1. Field of the Invention
The present invention relates to an exhaust gas purification device of an internal combustion engine.
2. Description of Related Art
Today, awareness of environmental conservation is increasing and excellent exhaust gas purification performance of an internal combustion engine is required. Specifically, for further spread of diesel engines, removal of exhaust particulates (i.e., particulate matters) such as black smoke discharged from the engine is important. In many cases, a diesel particulate filter (DPF) is equipped in an exhaust pipe for removing the particulate matters.
Since the DPF collects the particulate matters in the exhaust gas, a major part of the particulate matters is removed. However, the DPF will be clogged if the particulate matters only continue to deposit in the DPF. Therefore, it is necessary to regenerate the DPF by combusting and removing the deposited particulate matters. In order to combust the particulate matters deposited in the DPF, methods such as a post-injection are used. The post-injection injects fuel in a cylinder after a main injection.
If temperature increases excessively during the DPF regeneration, problems such as melting of the DPF or breakage of the DPF can occur. For example, if no-injection operation (i.e., operation of injecting no fuel into an engine) occurs when the temperature of the DPF is high during the regeneration of the DPF and the particulate matter deposition quantity in the DPF is large, an intake quantity decreases rapidly, so transmission of a heat inside the DPF to a downstream portion by exhaust gas is hindered. Accordingly, the heat stays and accumulates inside the DPF, thereby increasing the risk of the excessive temperature increase of the DPF. For example, the no-injection operation occurs during transition of an operation state of the engine from a normal operation state (i.e., a non-idle operation state) to an idle operation state or occurs when an engine brake is used while a vehicle is running on a downhill.
For example, Patent document 1 (JP-A-2003-27921) describes a technology that opens an intake throttle valve to increase a flow rate of the exhaust gas flowing into the DPF and to cool the DPF quickly under certain circumstances where there is a risk of the excessive temperature increase, thereby avoiding the excessive temperature increase of the DPF. Patent document 2 (JP-A-2002-188493) describes a technology that reduces a fresh air quantity by narrowing an intake throttle valve and by fully opening an EGR valve of an EGR pipe recirculating the exhaust gas. Thus, a combustion reaction of particulate matters in the DPF is suppressed to avoid the DPF excessive temperature increase.
An example of temporal transition of the temperature TDPF of the DPF is shown in FIG. 8. A transition of the intake quantity G is also shown in FIG. 8. In FIG. 8, broken lines (MODE I, MODE II) show the cases of Patent documents 1 and 2 respectively, and a solid line (NORMAL MODE) shows a case of normal intake air control. The normal intake air control means a case where opening degrees of the intake throttle valve and the EGR valve as of deceleration specified for each of individual devices are used. The control according to Patent document 1 is referred to as an intake air control mode I (MODE I, in FIG. 8) and the control according to patent document 2 is referred to as an intake air control mode II (MODE II, in FIG. 8) hereinafter.
In the example of FIG. 8, the DPF regeneration is started at time t1. After the no-injection operation (NO-INJECTION, in FIG. 8) starting from time t2, the operation state is changed to the idle operation state (IDLE, in FIG. 8) at time t3. As shown in FIG. 8, the DPF temperature TDPF increases after the DPF regeneration starts at the time t1. The intake quantity G decreases after the time t2. FIG. 8 shows the case where the particulate matter deposition quantity in the DPF is large.
Therefore, in the normal control shown by the solid line (NORMAL MODE), the large volume of the deposited particulate matters combusts at once after the time t2, and the heat inside the DPF stops moving downstream due to the reduction in the intake quantity G. As a result, the temperature TDPF of the DPF starts rising and eventually exceeds a temperature increase limit (LIMIT, in FIG. 8). The temperature increase limit means temperature, above which the melting or the breakage of the DPF can occur.
On the other hand, the intake air control modes I, II shown by the broken lines (MODE I, MODE II) exert the effect explained above and suppress the temperature increase of the DPF. Accordingly, the temperature TDPF does not exceed the temperature increase limit. Moreover, the response of FIG. 8 indicates that the intake air control mode I increases the intake air to move the heat in the DPF downstream, thereby quickly cooling the DPF. Moreover, it is indicated that the intake air control mode II suppresses the combustion inside the DPF, thereby relaxing the temperature increase.
The method of Patent document 1 is effective when engine rotation speed is relatively high. By increasing the flow rate of the exhaust gas, the temperature inside the DPF moves downstream and the risk of the excessive temperature increase of the DPF is reduced. However, the inventors of the present invention have knowledge that the method of Patent document 1 is not effective when the engine rotation speed is low. That is, a gas volume enough to pass the heat of the DPF downstream cannot be acquired when the engine rotation speed is low, so sufficient cooling effect cannot be acquired and the excessive temperature increase cannot be suppressed.
The method of Patent document 2 is effective when the engine rotation speed is low. By causing a state of oxygen deficiency, the risk of the excessive temperature increase of the DPF can be reduced. However, according to the knowledge of the inventors, if the method of Patent document 2 is used when the engine rotation speed is high, there can occur a problem that negative pressure in the cylinder in an intake stroke becomes excessive and oil loss via the piston ring from a cylinder wall surface occurs.
Thus, the methods of Patent documents 1 and 2 have advantages and disadvantages respectively. If the suitable method can be selected from among both methods in accordance with the situation, the problem can be avoided by utilizing the advantages of the both methods. However, such the technology has not been proposed in conventional technologies including Patent documents 1 and 2.